1. Field of the Invention
This invention relates to an improved infrared seeker assembly, and more particularly to one having an improved focal plane platform construction.
2. Discussion
Infrared detection systems are often used in conjunction with munitions and night vision systems for sensing electromagnetic radiation in the wavelength range of one to fifteen micrometers. Because many such detection systems have detector arrays which are most sensitive when operated at about 80.degree. K., a cooling system is required to produce and maintain the required low operating temperatures. Typically, such cooling systems either take the form of a cryostat utilizing the Joule-Thompson effect, or a Stirling cycle cryoengine. The cooling systems are used in conjunction with an evacuated dewar in which the electromagnetic detector is placed. The dewar is evacuated to remove thermally conductive gases which would otherwise occupy the area surrounding the detector so that potential heat loss through convection and conduction is minimized. The evacuated dewar also prevents moisture from condensing on the detector. The dewar is cooled by placing an indented region ("coldwell") of the dewar in contact with an expansion chamber ("expander") of the cryogenic cooling system. Commonly, the expander has a cylindrical tube ("coldfinger") having an end which is cooled and which supports a focal plane platform upon which the detector and related components are mounted. Alternately, the dewar can be constructed without a coldfinger such that the detector is mechanically supported directly by the focal plane platform. The cooling system produces cyclical cooling by sequential compression of a working fluid such as helium, removal of the heat generated during compression of the working fluid, and subsequent expansion of the working fluid. Thermal energy is withdrawn from the detector through the focal plane platform which is in thermally conductive communication with the cooling system. Since the cooling system is in thermal communication with the focal plane platform, expansion of the working fluid within the coldwell causes thermal energy to be withdrawn from the detector.
In order to produce efficient conductive withdrawal of thermal energy from an electromagnetic detector, the focal plane platform on which the detector is mounted must be fabricated from a material, or composition of materials, possessing specific metallurgical properties. Ideally, these properties include high strength, a high modulus of elasticity and high thermal conductivity. Additionally, the focal plane platform must produce low thermal distortion characteristics to minimize premature detector failures.
A number of design constraints affect the design of the focal plane platform. Since the focal plane platform is a structural support member, it must have sufficient bending stiffness to minimize mechanical deflection of the electromagnetic detector. Such requirements become particularly significant when the infrared seeker assembly is used as part of munitions subjected to intense vibration and high levels of boost-phase acceleration. Another significant design parameter is the extent to which heat is transferred through the focal plane platform. Another design consideration is the cool-down rate for the cold end components of the infrared seeker assembly. Since infrared seekers are often used in expendable munitions which must acquire a target soon after (or before) their launch, cool-down time becomes a critical consideration for some applications.
Prior art focal plane platforms have been fabricated from various materials. Titanium, copper and beryllium have been used, but unfortunately do not yield low thermal distortion characteristics under thermal cycling conditions. While other commercially available materials continue to be evaluated, none provide a coefficient of expansion substantially similar to that of typical hybrid detectors which is crucial to minimizing thermal distortion.